When converting PVC, i.e. polyvinyl chloride, the resin ordinarily undergoes a fluxing step after which it is processed to its desired shape. It is, of course, necessary that the resin remain stable at the high temperatures which are encountered during these processing and fluxing procedures. Additionally, the resin must yield under stress, it must have adequate flow properties and it must lend itself to milling and extrusion. Since unmodified PVC resin does not always possess all of these properties, it has been necessary or desirable to add various process aids to the resin. For instance, acrylic ester polymers in granular form are dry-mixed with the PVC resin and then milled on a calender or extruded. Alternatively, it is suggested by British Pat. Nos. 1,062,308 and 1,015,334 that the vinyl chloride may be polymerized in the presence of the pre-formed acrylic ester polymer or the acrylic ester monomer may be added to the polyvinyl chloride latex and polymerized in situ. The resulting product is an emulsion grade polyvinyl chloride blended with the polyacrylate ester modifier. Such modified polymers and processing aids permit faster calendering with improved gloss, better surface qualities on extrusion, freedom from plating and other benefits.
In copending application Ser. No. 43,568, filed June 4, 1970, the entire disclosure of which is incorporated herein, there is decribed an improved process for preparing a process aid for polyvinyl chloride resins. This novel process aid comprises a polyacrylate-modified PVC, i.e. a polyacrylate-modified polyvinyl chloride, which is prepared by: (1) suspension polymerizing vinyl chloride under conditions whereby polyvinyl chloride is obtained in a certain particle size range by means of a conventional, free radical initiated, suspension polymerization at a controlled rate of agitation and in the presence of a specified concentration of a suspending agent; (2) unreacted vinyl chloride is removed from the system after polymerization is at least 60% complete; (3) there is then added to the system an effective concentration of a chain transfer agent and a minor proportion, i.e. up to about 50% by weight of total solids of an acrylate ester monomer comprising methyl methacrylate and up to about 25% of its weight of one or more optional comonomers, as hereinafter defined, said monomer or monomers having first been pre-mixed with a fresh supply of an effective concentration of a monomer soluble, free radical catalyst; (4) the polymerization is then continued until the thus added methyl methacrylate, and any optional comonomers added therewith, are polymerized in and/or on the particles of the previously polymerized PVC; and, (5) the polyacrylate-modified polyvinyl chloride thereby obtained is then separated from the reaction medium. Apparently, the thus added acrylate ester monomer, i.e. the methyl methacrylate and any optional comonomers, are absorbed by and polymerized in and/or on the initially prepared polyvinyl chloride particles so as to thereby produce particles of a polyacrylate-modified PVC resin which acts as an effective process aid for blending with conventionally prepared polyvinyl chloride resin.
Subsequent to the filing of the above described copending application, it was found that when mixtures of PVC resins with this new polyacrylate-modified PVC process aid were subjected to various forming procedures, such as extrusion, calendering and injection molding, the surfaces of the resulting formed products were often characterized by their dullness and roughness. The latter properties are, of course, highly undesirable since, in order to be commercially acceptable, formed PVC resin products such as pipe, sheeting and film must have surfaces which are both smooth and glossy.
Thus, it is the prime object of this invention to find a means for preparing formed products from blends of a PVC resin and the above described polyacrylate-modified PVC process aid which is capable of yielding these formed products wih surfaces which are exceedingly smooth and glossy. Various other objects and advantages will become apparent during the course of the following disclosure.